Polaris Newbie with SL750 engine

Hi guys. I originally posted this in the tech section and was told that it doesn't see much action, so I'm reposting it here. I'm going to put my original thread and the very helpful reply I got from K447 below.

If anyone has any other suggestions or sources for parts, that would be great to know. I guess if I wanted to go the most direct route, a new crank complete assembly from SBT for $245 seemed pretty darn hard to beat. I have a machinist friend with a hydraulic press, but since K447 says there are special techniques needed to rebuild the crank, and all the bearings are probably suspect anyway, I don't know how I would end up being much cheaper piecemealing it together.

I'd still need one piston and jug, but I've seen nice condition used ones on ebay for $65, so I might gamble on that.

I've still got a couple of other open concerns. First, is my 125-130 psi in the other two cylinders acceptable? The gaskets have been sitting open in the sled for over a year, so I don't know how much they've absorbed water and expanded and possibly lowered the compression. I obviously haven't put any new gaskets on it yet; I just wanted a quick check. Is it likely with new gaskets and torqued, I'd get a little compression improvement?

Then, I 'd have to wonder about the small crack in the upper crankcase where the rod smashed into it. If this is not a high stress area, would I probably be OK with welding it back closed? It doesn't look like anything else is out of place. I learned something new reading the forums that I should torque the exhaust to the cylinders before you torque all the jugs down to the case, to avoid misalignment and leaks. So if reusing this is not a good idea, maybe I should pick up the other guy's block I mentioned below for $150 and use that case.

I won't know if his jugs are any good, since he has no starter or flywheel on it, so I can't immediately check compression, unless I pull mine completely off and put it on his block. I'd do that anyway, before I considered using any of them, but I'm obviously not able to do that before I buy it.

I still would like to try and diagnose what blew the PTO cylinder anyway before we get it going again. The rod is completely bent now, so I don't know if it ingested any water and hydrolocked it, causing the rod to bend. And then I assume that would change the timing on that one cylinder. Is that a possible way that it end up detonating?

K447 said the oil delivery was reliable. I was only planning on blocking it off because SBT recommended it on their shortblock instructions. And I've heard so many horror stories about oil injection systems failing. When I pulled the carbs, each line looked like oil was getting there.

I'm going to have to go through each carb as well I guess before I turn it over. I was planning to leave them stock as far as jetting, since this was for my wife and speed was not an issue. I've rebuilt several BN's, but not a super BN, which I think these are.

And should I go with a triple outlet fuel pump and where does anyone recommend getting it? I haven't checked if this thing is missing the return restrictor in the line yet. I assume in the service manual I can get the factory popoff settings and check it with a homemade tester.

I'm not likely to spend the $100 or $200 to get a used MFD display for my wife. I'd probably just replace the hole with a gas gauge. I assume the overtemp is just a switch so I can drill a hole in the dash and mount a bright LED or use a buzzer for that. She can live without the speedo and tach since I'm cheap.

Well, sorry to be so wordy and lengthy. I just have so many projects going on at once that I like to find out all the information I can about something new and get organized before I get started on another one.

The original posting and K447's reply is below:

Thanks,
John

Hi folks,

I hope I can get some advice. I picked up a 1994 SLT 750. First, not being familiar with Polaris too much what was the difference from the SL750 and SLT750?
Mine has reverse lever, but no power trim.

Anyway, I got it priced pretty right, so I don't think I can go too wrong. I thought it would make an OK sled for my wife, who is a very casual rider.
I prefer standups. This guy's wife was riding it and it just "died" on them. So he took it to a shop and they diagnosed PTO cylinder rod was broken.
They pulled the jug off and the large end was in pieces. Took a small chip off the piston, the very bottom of the sleeve.

It obviously had been detonating. The top of the piston was a spattered mess and bits of aluminum were sprayed on the surface of the combustion chamber.
So I'm assuming it had leaned out. I read that one of the carbs on these got low of fuel. I don't think this looks stock, but the P.O. (or his mechanic; he claimed to know
absolutely nothing about it) had put a "Tee" from the output of the single fuel pump. That "Tee" lead to two lines. Each one of those lines went to another "Tee" between
the fuel lines of each carb, i.e. between 1 and 2 and 2 and 3. So it would look to me like it would be pretty hard with this setup to have an uneven fuel flow.

I will need a new piston/rod at the min. The cylinder wall actually looks pretty good, but I have my doubts of course, after what it was through. They took off the other two heads and those looked very nice. I quickly slapped the heads back on and got a nice steady 125 and 127 on the other two cylinders.

Is this good enough for this motor?

The upper case has a small crack where the rod it it. It does not look like it would be hard to weld up. It does not look like a high stress area, but you tell me if I'm wrong.
I have used those little special alloy aluminum rods to make repairs such as this with good success, but I don't know.

So I know I will need to replace the large crank pin as it is messed from the rod, and the two (thrust bearing shims?) that go on each side of the large bearing.
I did not see those online in the SBT store, only the small roller bearing for the piston end. I really wanted to keep this on the cheapo, since it is for my wife, ha ha.
So I wished I would not have to spend the $245 for SBT's rebuilt crank, but I don't know where else to get those parts offhand. Since I also need at least a piston and a possible jug as well.

Strangely, a local guy on C/L has a 1995 engine that he pulled out of a sled that I can probably get for around $150. He said it was running when he pulled it, but it had a loud bearing noise, so it would need at least one bearing or possible more as well. He thinks the compression was good on all cylinders, so I'm thinking of just swapping one of those entire piston/rod/jug onto mine, and probably using his upper crankcase to eliminate the crack concern.

So I thought this seemed like the cheapest way to go. I know cheap is cheap, but my wife rides very little, and I'm hoping with her moderate speeds I can get her a few nice seasons out of this. She won't ride anything of mine.

Is it likely that if this guy's engine has one bad bearing that they're all shot? Or is it probably only the PTO end that commonly blows on these, like mine did? So maybe I can use one of the other bearings on the crank from his and mix and match. I'm assuming they are all the same, front to rear, but have not looked that up yet.

A full gasket set looks like another $100 bucks so I'm getting away from my lowball on this with just the minimal repairs.

The rest of the sled is in real pretty shape. The only thing they trashed was the MFD, which was taped over. Those look to be $100-150 used, but I assume since he was running without it, I can as well. She uses it so little, that I can easily check fuel level in between runs.

I'm planning on dumping the oil pump and pre-mixing it anyway, so the oil level sensor won't be of use to me.

What does anyone think? Am I being way to cheap for her, or can I make it work? I've admittingly done worse things in my life.

Thanks,
John

I also have a second question about the MFD pinout. Is it a bunch of separate (it's 8-wire) signals that I could just use the hole in the dash to at least put in a standard gas gauge or speedo? And just use one of the set of wires? Or are they multiplexed, etc? I have no wiring diagrams yet.

Thanks again, J.

-K447

Welcome to the Hulk John

SLT is a 3 seat model, SL is a two person model.
SLT has a longer hull, longer seat, and reverse.

The Polaris crank shafts are a press fit design. It requires a large hydraulic press and specialized knowledge to rebuild the crank.

If you are having the crank rebuilt, it doesn't make much sense to leave two crank bearing old sections in place, since all the bearings are likely to have similar wear and stress. The remaining rod bearings are probably nearing the end of their life too.

So you are looking at rebuilding that crank, or buying a good crank.

You would need to do a thorough inspection of that 'noisy' crank to determine if it was salvageable with just replacing the end bearings, or if it also needs a full rebuild.

The MFD connections are standard only for the fuel and oil lever senders;
33 ohms = full tank
240 ohms = empty tank

The RPM feed on the Yellow wire is Polaris specific. It carries a 12 volt DC bias with an AC RPM signal.
100Hz = 1000RPM

The Polaris oil pumps tend to be reliable. Just change the oil and fuel hoses, new filters, and make sure all the clamps are in good condition.